I successfully logged into my GMail account on Friday but today, when I tried logging, it says that the account password doesn't match.If you suspect that someone else has changed your GMail password, type in your GMail username (without @gmail.com) here [Forgot Password] and click Submit.
Please advise whom to contact and how to get my g-mail account back.
Google will send password resetting instructions to the secondary email address you provided during GMail signup.
If you don't have a secondary email address, try the same link again after five days. At that point, you'll be able to reset your password by answering the security question you provided when you created your account.
Though there is little you can do in situations when cannot answer the GMail security question and do not have access to the alternate email address, here's the link to contact GMail support. (choose Login/Account access from the topic list)
Resources: Google Accounts Help | Ask Google to Investigate